In recent years, fuel cells have been attracting attention as high-efficiency energy conversion devices. Fuel cells are roughly classified into two categories based on the type of the electrolyte used: low-temperature operating fuel cells, such as alkaline fuel cells, solid polymer electrolyte fuel cells, and phosphoric acid fuel cells; and high-temperature operating fuel cells, such as molten carbonate fuel cells and solid oxide fuel cells. Among them, the solid polymer electrolyte fuel cell (PEFC) that uses an ionically conductive polymer electrolyte membrane as an electrolyte has been receiving attention as a power supply source for stationary use, automotive use, portable use, etc., because it is compact in construction, achieves high output density, does not use a liquid for the electrolyte, can operate at low temperatures, and can therefore be implemented in a simple system.
The basic principle of the solid polymer electrolyte fuel cell is that, with gas diffusion electrode layers disposed on both sides of the polymer electrolyte membrane, whose anode side is exposed to a fuel gas (hydrogen or the like) and whose cathode side to an oxidizer gas (air or the like), water is synthesized by a chemical reaction occurring across the polymer electrolyte membrane, and the resulting reaction energy is extracted as electrical energy. Since the oxygen reduction reaction occurring as a side reaction at the cathode of the solid polymer electrolyte fuel cell proceeds while producing hydrogen peroxide (H2O2) in the process, there is concern that the electrolyte constituting the cathode electrode layer and the polymer electrolyte membrane adjacent to it may suffer degradation due to the hydrogen peroxide or peroxide radicals generated at the cathode electrode layer. At the anode also, if a phenomenon (crossover) occurs in which oxygen molecules permeate the polymer electrolyte membrane from the cathode side, hydrogen peroxide or peroxide radicals may likewise be generated, which can lead to the degradation of the electrolyte constituting the anode electrode layer.
To prevent the degradation of the polymer electrolyte membrane due to the peroxide generated at the electrode layers, it is known to provide a high durability solid polymer electrolyte in which a transition metal oxide having a catalytic ability to decompose the peroxide on contact, such as, among others, manganese oxide, ruthenium oxide, cobalt oxide, nickel oxide, chromium oxide, iridium oxide, or lead oxide, is dispersed through the polymer electrolyte membrane (Japanese Unexamined Patent Publication No. 2001-118591). It is also known to provide a sulfonic acid group-containing polymer electrolyte membrane for use in a solid polymer fuel cell, in which fine particles of a hardly-soluble compound of cerium are added into the polymer electrolyte membrane in order to increase its resistance to hydrogen peroxide or peroxide radicals (Japanese Unexamined Patent Publication No. 2006-107914).
As an electrolyte membrane for use in a solid polymer fuel cell, there is also proposed a polymer electrolyte membrane that contains cerium ions or manganese ions and that is reinforced by a porous membrane, etc., in order to enhance durability against hydrogen peroxide or peroxide radicals while also increasing the mechanical strength of the electrolyte membrane (Japanese Unexamined Patent Publication No. 2007-95433). The electrolyte membrane disclosed in Japanese Unexamined Patent Publication No. 2007-95433 can be produced by a method (1) in which a polymer compound membrane reinforced by a reinforcing structure is fabricated by molding and the membrane is then immersed in a solution of cerium ions, etc., a method (2) in which cerium ions, etc., are added in a solution of a polymer compound, which is then formed, together with a reinforcing structure, into the shape of a membrane by casting, or a method (3) in which cerium ions, etc., are added in a solution of a polymer compound, which is then formed into the shape of a membrane by casting, and the resulting membrane is placed on at least one side of a reinforcing structure and laminated together under heat and pressure.